Magnetic nanoparticles with large surface to bulk ratio is a growing area of interest. Considering the potentially large area of application of magnetic nanoparticles, as filler materials of various polymer materials, it can easily be understood that their relatively poor representation in comparison to micron-sized filler materials in polymers is an effect of the difficulties related to the processing of high-surface area nanoparticles. The explanation mainly lies in the fact that large surface areas also brings problems in achieving evenly distributed nanoparticle systems due to the favoured particle-particle interaction in comparison to particle-polymer/liquid interactions. The result is often severe agglomeration and aggregates of nanoparticles. The agglomerates in turn affect many macroscopic properties, such as mechanical, optical and magnetic etc. since these properties on a macroscopic scale are affected by the degree of close interaction at the nano scale level. In order to exploit the effects of nano-sized magnetic nanoparticles employed as fillers in organic matrix materials, the control over dispersion is therefore an unavoidable prerequisite.
Agglomeration of magnetic nanoparticles can be explained as a result of short-range isotropic forces and long-range anisotropic forces as well as poor compatibility between the surface of the inorganic nanoparticles and the matrix material. Different strategies to eliminate or reduces the interactive forces have been presented in literature. For example; It was reported that the primary reason of agglomeration in a non-polar solvent system can be explained as a relation to the strong influence of short distance van der Waals attractions, provided the dipole-dipole interactions between the particles were comparatively weak Lalatonne et al. (Nature Materials vol 3, February 2004). Lalatonne et al. demonstrated that the creation of agglomerates (clusters of nanoparticles) of 10 nm maghemite could be overcome to great extent by creating a non-polar spacer of sufficient number of carbon atoms between the nanoparticles, which reduced the interactions to great extent in a nonpolar solvent.
On the contrary, as particle sizes are increased and dipolar forces become dominant, it has been shown that abrupt transitions from separate particles to randomly oriented linear agglomerates/aggregates appears. However, as the majority of liquids and polymer systems exhibit various degree of polarity, the dispersion of magnetic nanoparticles is not simply solved by surface modifying the nanoparticles with one coating due to the fact that the surface coating with optimal solubility for a specific liquid polymer system is not the same for different system with different chemical character. In addition, the coating procedure of the magnetic nanoparticles can potentially make dispersion even more difficult due to the potential risk of obtaining coated agglomerates rather than single particles. Agglomerates are quickly formed when producing a magnetic material by precipitating transition metal salts already in the precipitation phase due to the magnetic forces between the particles. Considerably stronger agglomerates are formed when the particles are dried, i.e. it is extremely difficult to distribute these particles in polymers. Thus, the unique features relating to the particles being in the nano-scale range (1-200 nm) and that they consist of individual crystals can not be taken advantage of since functional material properties related to the nano-scopic dimension are negated.
Agglomeration is a major problem since the magnetic properties of a hybrid material on a macroscopic scale depend on the degree of agglomeration and is related to the degree of exchange coupling and dipolar forces between particles in the hybrid material. Coating the nanoparticles has also been investigated. But there are several problems with this technique such as not only one coating works in all situations for different polymers and no solution has been suggested how to optimize the coating in respect of the polymer matrix to have an optimal solubility.
US2007090923 suggests that aminated groups bound to the surface can be used to prevent agglomeration.
E. Sourty et. al. (Ferrite-loaded membranes of microfibrillar Bacterial Cellulose Prepared by in Situ Precipitation. Chem. Mater., Vol. 10, No. 7, 1998), relates to microfibrillar bacterial cellulose prepared in situ and discusses that the preparation of a uniform nanocomposite is extremely difficult. It is suggested to apply paper fabrication technology using micofibrils with attached ferrites. However, the transmission electron micrograph illustrates that the agglomeration problem persists.
US20050245658, discloses a wettable polymer having ion-exchangeable groups to form metal oxides trapped within the polymer structure. Further, a technique for the synthesis of magnetic nanocomposites is disclosed where the polymers have ion exchangeable groups attached.
Yano et al. (Optically transparent composites reinforced with networks of bacterial nanofibres, Adv. Mater. 17, 153-155, 2005), has shown that dried bacterial cellulose can be impregnated with resins such as epoxy, acrylic, and phenol-formaldehyde under vacuum. The new material increased in weight and got new physical properties. The resin used by Yano et al. resulted in a mechanically stable and flexible transparent material. By using a similar approach as Yano et al., a new magnetic nanoparticle containing material can be made.
Therefore there is a need within the technical field of magnetic nanoparticle cellulose material to solve the agglomeration problem. Especially, since the agglomerates in turn affect many macroscopic properties, such as mechanical, optical and magnetic etc. since these properties on a macroscopic scale are affected by the degree of close interaction at the nano scale level.